High bit-rate services such as multimedia transmission will result in frequency-selective fading and inter-symbol interference (ISI). The conventional technique to reduce ISI and the effects of frequency selective fading is to equalize in the time-domain (TD) in W-CDMA, for example. For broadband transmission, the complexity of equalization in TD could be very high because of the large number of channel impulse responses within the spectrum band. Multicarrier (MC) CDMA is an orthogonal frequency-division multiplexing (OFDM) scheme which divides the entire bandwidth into multiple narrow-band subcarriers and implements the spreading operation in the frequency domain (FD). See, for example, Hara et al. (“Overview of Multicarrier CDMA”, IEEE Communications Magazine, pp. 126–133, December 1997). MC-CDMA is a promising technique to eliminate ISI and the effects of frequency selective fading. Furthermore, it just needs one-tap equalization due to the flat fading in each narrowband subcarrier. However, it has severe disadvantages such as difficulty in subcarrier synchronization and sensitivity to frequency offset and nonlinear amplification. In an MC-CDMA system, Peak-to-Average Ratio (PAR) and frequency offset degrade the system performance.
Single-carrier modulation, which uses broadband equalization in the frequency domain, has been shown to have many advantages over multicarrier modulation. The single-carrier modulation systems have lower peak-to-average power ratio than multicarrier modulation systems. In particular, the single-carrier direct-sequence (DS) CDMA system with cyclic prefix (CP) has been proposed for broadband communications. As shown in FIG. 1, the cyclic prefix (CP) in the conventional CP-CDMA transmitter is added to the W-CDMA signals in the chip level. A plurality of transmitted symbols du[n] are upsampled and filtered by assigned spreading codes Cu[n]. After the power in each code channel is allocated, the spread symbols of different code channels are summed up. Then, a serial-to-parallel converter is used to split the data stream into NK parallel samples. The last L chip samples of the data block are copied and added in front of the data block as CP, as shown in FIG. 2. The CP added data blocks are converted in a single data stream by a parallel-to-serial converter for transmission.
At the receiver side, after CP is removed from the received signal, the signal is converted into a plurality of parallel streams by a serial-to-parallel converter and transformed into frequency domain by FFT (fast Fourier transform) operation, as shown in FIG. 3. The channel is equalized in the frequency domain and the equalized signals is transformed into time domain by IFFT (Inverse FFT). The output of the IFFT is converted to a single data stream by parallel-to-serial conversion and despread. The despread signal is fed to the channel decoder. The detailed description of the conventional CP-CDMA transceiver can be found in Baum et al. (“Cyclic-Prefix CDMA: An Improved Transmission Method for Broadband DS-CDMA Cellular systems” WCNC2002, Vo. 1, pp. 183–188) and Vook et al. (“Cyclic-Prefix CDMA with Antenna Diversity”, VTC Spring 2002, IEEE 55th, Vol. 2, pp. 1002–1006).
The CP-CDMA system can be directly applied for current 3G W-CDMA systems by adding CP to the conventional W-CDMA signals in chip level. As such, the required modification on the transmitter side is negligible. However, the major drawback of the CP-CDMA is that the conventional receivers equalize the frequency domain and despread the signal in the time domain separately. Such a system does not give the optimum solution in the MMSE (Minimum Mean Square Error) criteria.
It is advantageous and desirable to provide a method and device for improving the CP-CDMA system performance.